This invention relates to a measuring set-up, especially for determining the concentration of substances that are preferably contained in sample vessels, e.g., biological liquids, comprising at least one electrochemical, optical, calorimetric, ion-selective, or similar such sensor, with at least one thin membrane being placed between the substance to be analyzed and the sensor, or rather the active surface of the sensor. The membrane forms part of the sample vessel and is permeable for a species or measurement variable (such as the ionic concentration) depending on the particular substance and its properties, especially its concentration, and through which membrane the active sensor surface may be brought into contact with the substance, and which membrane will cover or enclose the substance to be analyzed and will constitute at least one wall of the sample vessel containing this substance, the active sensor surface itself not carrying a membrane.